1. Field of the Invention
The present invention relates to a rolling member and, in particular, to a rolling member of a rolling bearing used in a cylinder head, rocker arm, and the like of an engine for automobiles and auto-bicycles which is operated in a high engine speed range.
2. Description of the Art
Recently, there has been an increasing trend to change the bearing system of a cylinder head and a rocker arm of an engine for automobiles and auto-bicycles from a conventional slipper system to a rolling system to reduce friction of the engine and as a countermeasure for wear of the cam.
In this case, for an outer race of a rolling bearing, a conventional bearing steel type 2 (SUJ - 2) is used and the outer race was formed by turning a bar material and thereafter by hardening through quenching.
The rolling members which are used for such engines are required to provide high reliability and semipermanent service life from the standpoint of improving the safety and durability of the engines.
Accordingly, in order to meet such requirements, the Applicants of the present application proposed a long life rolling member in Japanese Patent Laid-Open Publication No. Hei 2-125,841, and Japanese Patent Laid-Open Publication No. Hei 2-209,452.
In such prior art rolling members, in order to reduce non-metallic inclusions which serve as a starting point of a crack at the time of plastic working, the cold forgeability is improved by reducing the content of S (sulfur) which is an element acting to produce the non-metallic inclusions and, at the same time, a long service life is attained.
Furthermore, a rolling bearing which provides longer service life has been proposed in Japanese Patent Laid-Open Publication No. Hei 2-125,838, in which the stress concentration is moderated to suppress the flanking due to fatigue by adding Nb (niobium) and V (Vanadium) and by forming fine crystal particles.
In engines for automobiles and auto-bicycles, since they are often operated at high engine speeds exceeding 10,000 revolutions per minute, the rolling bearing is also required to provide characteristics which enable it to withstand such severe operation.
However, in the prior art rolling bearing and rolling member, in a high engine speed region in which a phenomenon of jumping of the cam and the rocker arm exists, the follow of the rolling bearing to the rotation of the cam becomes unsatisfactory and a fatigue break (crack) of an outer race occasionally occurs. This fatigue break is caused by the presence of a non-metallic inclusion which acts as a starting point for a crack or fatigue break when a bending stress acting on the outer race becomes large. Accordingly, in order to achieve a long service life, it is necessary to reduce the fatigue break.